JP7172275B2 - Hot stamping die steel, hot stamping die and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot stamping die steel, a hot stamping die and a method for manufacturing the same.

In recent years, the need for ultra-high tensile strength steel sheets with a tensile strength exceeding 1 GPa has been increasing for the purpose of reducing the weight of automobiles and improving crash safety. However, when a steel sheet having a tensile strength of 1.2 GPa or more is formed by cold pressing, problems such as an increase in forming load and springback and formability occur. Therefore, hot stamping (also referred to as hot pressing or hot stamping) has recently attracted attention. In the hot stamping method, a steel plate is heated to the austenite temperature or higher, press-formed, held at the bottom dead center of the die, and rapidly cooled and quenched.

As an advantage of the hot stamping method, it is possible to obtain a molded product of an ultra-high tensile strength steel sheet having a tensile strength of about 1.5 GPa by quenching by die quenching that is rapidly cooled in a mold. It also has the advantage of being excellent in moldability, such as almost no springback.
However, the hot stamping method has a problem of low productivity. In other words, it takes time to hold the bottom dead center for die quenching, resulting in low productivity. As a countermeasure, a mold with high thermal conductivity is required. This is because, in die quenching, the heat of the steel sheet is absorbed by the die, and the higher the thermal conductivity of the die, the shorter the time required to maintain the bottom dead center and the higher the productivity.
Also, hot stamping molds are required to have high hardness in order to improve wear resistance.

Therefore, hot stamping die steel is required to have both high hardness and high thermal conductivity when formed into a die. In general, in order to obtain a high-hardness mold, it is necessary to increase the amount of alloy in the mold steel. There is a trade-off relationship. Therefore, the optimal chemical composition is being studied by controlling the alloying amount. For example, Patent Literature 1 and Patent Literature 2 propose a chemical composition of mold steel that achieves both hardness and thermal conductivity.

JP 2017-43814 A JP 2018-24931 A

The mold steels of Patent Documents 1 and 2 are useful for hot stamping. However, considering the quenching and tempering characteristics (tempering hardness) of mold steel and, in some cases, the working surface of hot stamping molds is nitrided before use, conventional mold steel In the case of steel, hardness may be insufficient.
An object of the present invention is to provide a mold steel capable of producing a mold having both high hardness and high thermal conductivity suitable for hot stamping, a mold for hot stamping, and a method for manufacturing the same. is.

In view of such circumstances, the present inventors have conducted intensive research, and as a result, have found the optimum mold steel for hot stamping by controlling the alloy content. Then, by using the above mold steel, the inventors have found a hot stamping mold capable of achieving high hardness and high thermal conductivity, and a manufacturing method thereof.

That is, in the present invention, in mass%, C: 0.45 to 0.65%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.6%, P: 0.05% or less , S: 0.01% or less, Cr: 2.0 to 4.0%, Mo: 1.2 to 3.2%, V: 0.3 to 0.8%, the balance Fe and impurities It is a hot stamping mold steel with

In addition, in the present invention, in mass%, C: 0.45 to 0.65%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.6%, P: 0.05% or less , S: 0.01% or less, Cr: 2.0 to 4.0%, Mo: 1.2 to 3.2%, V: 0.3 to 0.8%, the balance Fe and impurities A hot stamping mold having a hardness of 45 to 55 HRC and a thermal conductivity of 30 W/(m·K) or more. Preferably, the hot stamping mold has a nitride layer on the working surface of the hot stamping mold.

The present invention also provides a method for manufacturing hot stamping dies, wherein the die steel is quenched and tempered at a quenching temperature of 1020 to 1080°C and a tempering temperature of 600 to 650°C. Preferably, the method for manufacturing a hot stamping die further includes nitriding the working surface of the hot stamping die after performing the quenching and tempering.

ADVANTAGE OF THE INVENTION According to this invention, the optimal mold steel for hot stamping is obtained. Further, by using this mold steel, it is possible to provide a hot stamping mold having both high hardness and high thermal conductivity, and a method for manufacturing the same.

FIG. 2 is a graph showing the hardness at each tempering temperature for an example of a mold produced by tempering at 500 to 650° C. after quenching the mold steels of the invention examples and comparative examples. FIG. 2 is a graph showing the thermal conductivity of an example of a mold produced by tempering the mold steels of the invention examples and comparative examples to a hardness of 50 HRC after quenching. FIG. 2 is a graph showing the hardness at each tempering temperature for an example of a mold produced by tempering the mold steel of the present invention at 500 to 650° C. after quenching. FIG. 2 is a graph showing the thermal conductivity of an example of a mold produced by tempering the mold steels of the invention examples and comparative examples to a hardness of 45 HRC after quenching.

The feature of the present invention is that the hot stamping die is produced by quenching and tempering the die steel, and in some cases, nitriding the work surface thereof. In addition, it has been found that there is an optimum chemical composition of mold steel for simultaneously achieving high hardness and high thermal conductivity in hot stamping molds. In addition, the optimum quenching and tempering conditions for simultaneously achieving the above high hardness and high thermal conductivity have been identified. Each component of the present invention will be described below.

(1) The hot stamping die steel of the present invention contains C: 0.45 to 0.65% and Si: 0.1 to 0.1% by mass (hereinafter simply referred to as "%"). 4%, Mn: 0.1-0.6%, P: 0.05% or less, S: 0.01% or less, Cr: 2.0-4.0%, Mo: 1.2-3.2 %, V: 0.3 to 0.8%, the balance being Fe and impurities.

・C: 0.45 to 0.65%
C is an element that dissolves in the base (matrix) by quenching and improves the hardness of the mold. In addition, it is an element that forms carbides with carbide-forming elements such as Cr, Mo, and V, which will be described later, to improve the hardness of the mold. However, if the amount of C is too large, the toughness of the mold is lowered due to coarsening of primary carbides and the like. Therefore, C is set to 0.45 to 0.65%. Preferably it is 0.47% or more. More preferably, it is 0.49% or more. Moreover, it is preferably 0.63% or less. More preferably, it is 0.60% or less. More preferably, it is 0.58% or less.

・Si: 0.1 to 0.4%
Si is used as a deoxidizing agent in the smelting process. Further, it is an element that improves the hardness of the mold by dissolving in the base material. However, if the amount of Si is too high, the tendency of segregation in the steel becomes stronger after melting, and the solidified structure becomes coarser, leading to a decrease in the toughness of the mold. Therefore, Si should be 0.1 to 0.4%. Preferably it is 0.14% or more. More preferably, it is 0.17% or more. Moreover, it is preferably 0.35% or less. More preferably, it is 0.3% or less.

・Mn: 0.1 to 0.6%
Mn is used as a deoxidizing agent and a desulfurizing agent in the smelting process. Further, it is an element that contributes to strengthening of the base material and improvement of hardenability and toughness after quenching and tempering. However, too much Mn reduces the thermal conductivity of the mold. Therefore, Mn is set to 0.1 to 0.6%. Preferably it is 0.15% or more. Moreover, it is preferably 0.5% or less. More preferably, it is 0.4% or less. More preferably, it is 0.3% or less.

• P: 0.05% or less P is an element that can be unavoidably contained in steel. If too much P is added, it will segregate at the prior austenite grain boundaries during heat treatment such as tempering, deteriorating the toughness of the mold. Therefore, P is regulated to 0.05% or less. Preferably, it is regulated to 0.01% or less.

S: 0.01% or less S is an element that can be unavoidably contained in steel. If the amount of S is too large, the hot workability deteriorates during blooming of the steel ingot. Therefore, S is regulated to 0.01% or less. Preferably, it is regulated to 0.004% or less.

・Cr: 2.0 to 4.0%
Cr is an element that solid-dissolves in the substrate and increases the hardness. In addition, it is an element that increases hardness by forming carbides, and like Mo and V, which will be described later, is an element that contributes to secondary hardening during tempering.
At this time, Cr can increase temper softening resistance compared to Mo and V (that is, even if the tempering temperature is increased, the rate of decrease in hardness obtained by secondary hardening can be reduced. can) element. Generally, molds are adjusted to the desired hardness by quenching and tempering mold steel, but in order to increase the thermal conductivity of hot stamping molds, it is effective to raise the tempering temperature. be. In the present invention, by setting the Cr content to 2.0% or more, high hardness can be maintained even if the tempering temperature is raised (for example, even if it is 600° C. or higher). At the same time, the thermal conductivity can also be increased. For example, a hot stamping mold having a hardness of 45 to 55 HRC and a thermal conductivity of 30 W/(m·K) or more can be obtained.
In addition, by increasing the Cr content, it is possible to improve the nitriding properties (surface hardness of the nitrided layer) of the mold steel. In the case of treatment, the wear resistance (hardness of the working surface) of the mold can be improved.

However, if the Cr content is too high, it becomes difficult to increase the thermal conductivity of the mold due to the fact that the alloy content of the mold steel increases. Therefore, Cr is set to 2.0 to 4.0%. Preferably, it is 2.2% or more. More preferably, it is 2.4% or more. More preferably, it is 2.6% or more. Even more preferably, it is 2.8% or more. Moreover, it is preferably 3.8% or less.

・ Mo: 1.2 to 3.2%
Like Cr, Mo is an element that dissolves in the base material to increase the hardness, is an element that increases the hardness by forming carbides, and is an element that contributes to secondary hardening during tempering. be. It is also an element that improves hardenability. However, if the amount of Mo is too large, the thermal conductivity of the mold is lowered due to the fact that the amount of alloy in the mold steel is increased. Therefore, Mo is set to 1.2 to 3.2%. Preferably it is 1.5% or more. More preferably, it is 1.9% or more. More preferably, it is 2.2% or more. Even more preferably, it is 2.5% or more. Moreover, it is preferably 3.0% or less.

・V: 0.3 to 0.8%
V, like Cr, is an element that increases the hardness by forming carbides, and is an element that contributes to secondary hardening during tempering. However, if the amount of V is too large, the heat conductivity of the mold is lowered due to the fact that the amount of alloy in the mold steel is increased. Therefore, V is set to 0.3 to 0.8%. Preferably it is 0.4% or more.

Remaining Fe and Impurities Considering that the thermal conductivity of the mold decreases as the amount of alloy in the mold steel increases, it is preferable that the remainder other than the above element species is substantially Fe. However, elemental species not specified here (for example, elemental species such as Cu, Al, Ca, Mg, O (oxygen), N (nitrogen), etc.) are elements that may inevitably remain in the steel. , the inclusion of these elements as impurities is allowed. In addition, Ni is useful as an element species that contributes to the improvement of the toughness of the mold. should be kept low. And, preferably, 0.25% is allowed as the upper limit of Ni amount regulation.

(2) The hot stamping mold of the present invention has the above component composition, hardness of 45 to 55 HRC, and thermal conductivity of 30 W/(m·K) or more.
By quenching and tempering the mold steel having the above chemical composition, a mold having a hardness of 45 HRC or more can be obtained. This hardness is a value measured at room temperature (ordinary temperature). By setting the hardness of the mold to 45 HRC or more, it is possible to impart excellent wear resistance to the mold during use. Preferably it is 48HRC or more. More preferably, it is 50 HRC or more. At this time, it is not necessary to specify the upper limit of the hardness of the mold. However, in the case of mold steel with the above chemical composition, the secondary hardening peak is generally within the tempering temperature range of 500 to 600°C, and a thermal conductivity of 30 W/(mK) or more is achieved. When it is considered to raise the tempering temperature (for example, 600° C. or higher) in order to achieve this, it is realistic that the upper limit is about 55 HRC. It is preferably 53 HRC or less. More preferably, it is 51 HRC or less.

Further, by quenching and tempering the die steel having the above chemical composition, a die having a thermal conductivity of 30 W/(m·K) or more can be obtained. In addition, this thermal conductivity is a value measured at room temperature (ordinary temperature). By setting the thermal conductivity of the mold at room temperature to 30 W/(m K) or more, it is possible to maintain high thermal conductivity even when the mold is being used in the hot stamping method (for example, 100 to 400 ° C). . It is preferably 32 W/(m·K) or more. More preferably, it is 35 W/(m·K) or more. More preferably, it is 37 W/(m·K) or more. Such thermal conductivity can be easily achieved by increasing the tempering temperature to, for example, 600° C. or higher.
At this time, if the hardness of the mold can be maintained at 45 HRC or more, there is no need to specify the upper limit of the thermal conductivity of the mold. However, in the case of the mold steel having the chemical composition described above, it is expected that the hardness of the mold will decrease as the tempering temperature increases (for example, to 600°C or higher). Considering this, it is realistic that the upper limit of the thermal conductivity is about 50 W/(m·K) when the hardness of the mold is less than 45 HRC. It is preferably 47 W/(m·K) or less. More preferably, it is 45 W/(m·K) or less.

(3) The hot stamping mold of the present invention preferably has a nitride layer on its working surface.
As described above, the hot stamping mold of the present invention has a hardness of 45 HRC or higher and a thermal conductivity of 30 W/(m·K) or higher, which combines high hardness and high thermal conductivity. Further, since the working surface of the mold further has a nitride layer, the wear resistance of the mold (hardness of the working surface) can be further improved. The working surface is the surface of the mold that comes into contact with the steel plate during hot stamping.

(4) In the hot stamping die manufacturing method of the present invention, the die steel is quenched and tempered at a quenching temperature of 1020 to 1080°C and a tempering temperature of 600 to 650°C.
When quenching and tempering the mold steel having the above chemical composition, the quenching temperature can be approximately 1020 to 1080° C., although it varies depending on the target hardness and the like. It is preferably 1050° C. or less.
Then, the mold steel that has been quenched at this quenching temperature is tempered at a high temperature of 600 ° C. or higher, thereby maintaining sufficient hardness of the mold and increasing the thermal conductivity of the mold. For example, a mold having a hardness of 45 HRC or more and a thermal conductivity of 30 W/(m·K) or more can be obtained. At this time, it is realistic to set the upper limit of the tempering temperature to 650° C. in order to maintain the hardness of 45 HRC or more. It is preferably 640° C. or less. More preferably, it is 630° C. or less.

The die steel of the present invention is prepared into a hot stamping die having a predetermined hardness by quenching and tempering. During this time, the mold steel is shaped into the shape of the hot stamping mold by various machining such as cutting and drilling. The timing of this machining can be performed in a low hardness state (that is, in an annealed state) before quenching and tempering. In this case, finish processing may be performed after quenching and tempering. In some cases, the machining may be performed in a pre-hardened state after quenching and tempering, together with the finishing.

(5) In the hot stamping mold manufacturing method of the present invention, preferably, the working surface of the mold after the quenching and tempering is further subjected to nitriding treatment.
As described above, by quenching and tempering the mold steel having the above chemical composition, a mold having a hardness of 45 HRC or more and a thermal conductivity of 30 W/(m·K) or more can be obtained. Since the mold steel having the above chemical composition is also excellent in nitriding properties, the working surface of the mold after quenching and tempering is further nitrided to improve the mold. can improve the wear resistance (hardness of the work surface). At this time, as conditions for the nitriding treatment, various known nitriding treatments such as gas nitriding treatment and salt bath nitriding treatment can be applied.

A steel ingot of 10 kg having the chemical composition shown in Table 1 was melted. Then, this steel ingot was heated to 1160° C., hammer-stretched, and then allowed to cool. 10 and 11 (comparative examples) were produced.

<Evaluation of tempering hardness>
Die steels 1 to 5 and 10 and 11 were quenched at a quenching temperature of 1030°C. At this time, the cooling conditions were such that the half-cooling time was 40 minutes, assuming the cooling rate when the mold steel has the actual size of the hot stamping mold (the half-cooling time is the quenching temperature to (quenching temperature + room temperature)/2). Then, the quenched mold steel is tempered at a tempering temperature of 500 to 650 ° C., corresponding to mold steel 1 to 5, 10, and 11 in that order, which corresponds to a hot stamping mold. Molds 1-5 and 10, 11 were obtained. Tempering was performed twice and held at each temperature for 2 hours. The tempering temperature was set to 7 conditions in increments of 25°C. Then, for each of the molds 1 to 5, 10 and 11, the Rockwell hardness (C scale) of the central portion at room temperature was measured for each tempering temperature. The results are shown in FIG.

Molds 1 to 5 of Inventive Examples maintained high temper hardness over the entire tempering temperature range of 500 to 650°C. Even when the tempering temperature is raised to 600° C. or higher, which is considered to be particularly effective for increasing the thermal conductivity of the mold, a high temper hardness of 45 HRC or higher was achieved.
On the other hand, the mold 10 of the comparative example also maintained sufficient temper hardness over the entire tempering temperature range of 500 to 650°C. A hardness of 45 HRC or higher was achieved even at a tempering temperature of 600°C. However, when the tempering temperature was increased to 625°C, the tempering hardness fell below 45HRC. The mold 11 of the comparative example already had a temper hardness of less than 45 HRC when the tempering temperature was 575°C.

<Evaluation of thermal conductivity>
Based on the results of the above <Evaluation of tempering hardness>, molds 1 to 5 and 10 were tempered at a tempering temperature higher than the tempering temperature (approximately 500 to 600 ° C.) at which the secondary hardening peaked. The thermal conductivity was measured when the hardness was 50HRC. As for the measurement procedure, first, a metal mold was processed into a disc-shaped test piece of 10 mm in diameter and 2 mm in thickness, and the thermal diffusivity and specific heat of this test piece were measured by the laser flash method. Then, using the measured values of thermal diffusivity and specific heat, the thermal conductivity at room temperature was calculated from the following formula. The results are shown in FIG.
Thermal conductivity λ(W/(m・K))=ρ・α・C _p
(ρ: room temperature density, α: thermal diffusivity, C _p : specific heat)

Molds 1 to 5 of Inventive Examples achieved a high hardness of 50 HRC and had a high thermal conductivity of 30 W/(m·K) or more. And since the tempering temperature at this time is about 600°C (see Fig. 1), by further raising this tempering temperature, it is possible to maintain the tempered hardness of 45 HRC or more, and further improvement in thermal conductivity is expected. can.
In contrast, the mold 10 of the comparative example achieved a high hardness of 50 HRC, but the thermal conductivity was below 30 W/(m·K). When the tempering temperature at this time is less than 600° C. (see FIG. 1), if the tempering temperature is raised in order to improve the thermal conductivity, it becomes difficult to maintain a tempered hardness of 45 HRC or more.

A steel ingot of 10 kg having the chemical composition shown in Table 2 was melted. Then, this steel ingot was heated to 1160° C., hammer forged and then allowed to cool, and the steel material after this allowed to cool was annealed at 870° C. to produce mold steel 6 (example of the present invention). .

<Evaluation of tempering hardness>
Die steel 6 was quenched at a quenching temperature of 1030°C. At this time, as for the cooling conditions, the semi-cooling time was set to 40 minutes, assuming the cooling rate when the mold steel has the actual size of the hot stamping mold. Then, the quenched mold steel was tempered at a tempering temperature of 500 to 650° C. to obtain a mold 6 corresponding to a hot stamping mold. Tempering was performed twice and held at each temperature for 2 hours. The tempering temperature was set to 7 conditions in increments of 25°C. Then, the Rockwell hardness (C scale) of the central portion of the mold 6 at room temperature was measured for each tempering temperature. The results are shown in FIG.
Mold 6 of Inventive Example maintained high temper hardness over the entire tempering temperature range of 500 to 650°C. Even when the tempering temperature is raised to 600° C. or higher, which is considered to be particularly effective for increasing the thermal conductivity of the mold, a high temper hardness of 45 HRC or higher was achieved.

<Evaluation of thermal conductivity>
In addition to the mold 6, the mold 10 evaluated in Example 1 was also added. The thermal conductivity was measured when the tempering hardness was 45HRC. The measurement procedure was the same as in Example 1. The results are shown in FIG.
The mold 6 of the present invention example achieved a high hardness of 45 HRC and had a high thermal conductivity of 30 W/(m·K) or more. On the other hand, the mold 10 of the comparative example achieved a high hardness of 45HRC, but the thermal conductivity was below 30W/(m·K).

From the results of the above examples, the present invention can provide a hot stamping mold having high hardness and high thermal conductivity. By forming a nitride layer on the working surface of this hot stamping die, further improvement in wear resistance is achieved.

Claims (5)

% by mass, C: 0.45 to 0.65%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.6%, P: 0.05% or less, S: 0.01 % or less, Cr: 2.0 to 4.0%, Mo: 1.2 to 3.2%, V: 0.3 to 0.8%, the balance being Fe and impurities. Mold steel for hot stamping. % by mass, C: 0.45 to 0.65%, Si: 0.1 to 0.4%, Mn: 0.1 to 0.6%, P: 0.05% or less, S: 0.01 % or less, Cr: 2.0 to 4.0%, Mo: 1.2 to 3.2%, V: 0.3 to 0.8%, the balance being Fe and impurities, and the hardness is 45 to 55 HRC, and a hot stamping mold characterized by having a thermal conductivity of 30 W/(m·K) or more. 3. A hot stamping mold according to claim 2, characterized in that it has a nitride layer on its working surface. The hot stamping mold steel according to claim 1 is quenched and tempered at a quenching temperature of 1020 to 1080 ° C. and a tempering temperature of 600 to 650 ° C., and has a hardness of 45 to 55 HRC and a thermal conductivity of 30 W / A method for manufacturing a hot stamping mold, characterized by obtaining a hot stamping mold having a (m·K) or more . 5. The method of manufacturing a hot stamping die according to claim 4, further comprising nitriding the work surface after performing the quenching and tempering.

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